The worm eggs hatch inside the snail, where they form cysts (the white spots seen in the infected snail at top) and make the snail infertile. Both the infected and healthy snail (bottom) are shown with shells removed. (Gabe Hart/Indiana University)

Snails may be more motivated to reproduce with a partner when there are parasites around, a new study suggests.

The New Zealand mud snail can reproduce by cloning itself and seems to favour that type of reproduction in a parasite-free environment, said the study led by Canadian researcher Kayla King and published in the latest edition of Current Biology.

But when there is a chance of becoming infected with a parasitic worm — whose scientific name is, incidentally, Microphallus — it appears the snails turn to sex as an evolutionary battle tactic.

"Sexual reproduction results in genetically unique individuals," King said in an email. "These are more difficult for the parasites to adapt to."

Normally, a line of females that can reproduce asexually by cloning themselves will increase its population more quickly than a line of females that reproduces sexually and will soon dominate the entire population. That's because sexual reproduction requires males, who can't produce their own offspring directly. Because of that, biologists argued that there needs to be some factor that makes sex advantageous in order for it to win out over cloning.

The recent study supports a long-held hypothesis among evolutionary biologists that parasites do just that.

Microphallus causes infertility

Microphallus is a type of worm called a trematode whose eggs are found in duck feces in shallow areas of lakes. Snails that live there eat the eggs, and the worm larvae hatch inside them, forming cysts and rendering the snails infertile. Ducks that forage in the shallow parts of lakes eat the infected snails, and the worm completes its life cycle inside the duck, where it mates and lays eggs that are excreted in the duck's feces.

Because there are no ducks in the deeper parts of the lake, there are no worms there either. King, a PhD student doing research at Indiana University, supported by a scholarship from the Canada's Natural Sciences and Engineering Research Council, took snails from both shallow and deep areas of Lake Alexandrina and Lake Kaniere on New Zealand's South Island, which are separated by a mountain range but have the same types of snails and the same type of Microphallus parasite.

They noticed that among snails in deeper waters, there were very few males, and most females reproduced asexually by cloning themselves.

In shallow waters, the researchers found more males, suggesting that more sexual reproduction was happening.

The researchers found the deep-water snails or snails from a different lake were generally not very susceptible to becoming infected by the parasites. However, snails from the shallow parts of lakes were very susceptible to parasites from their own lake.

The researchers concluded that the parasites had evolved to better infect the snails. That was associated with sexual reproduction among snails, indicating that they too were evolving in response to the worms' evolution.